Poly(L-lactide-co-ε-caprolactone) and its use as material for medical applications has been studied extensively in the past. Grijpma et al. described the use of a semi-crystalline copolymer of L-lactide and ε-caprolactone (50/50) for bridging of peripheral nerve defects (Polymer Bulletin 25 (1991) 327). Den Dunnen et al. (J. Mat. Sci.: Mat. in Med. 4 (1993) 521-525), Aldini et al. (Biomaterials 17 (1996) 959-962) and Rodriquez et al. (Biomaterials 20 (1999) 1489-1500) reported that poly(L-lactide-ε-caprolactone) was highly biocompatible and that the use of poly(L-lactide-co-ε-caprolactone) nerve guides resulted in good functional nerve recovery. However, it was concluded that the low degradation rate of the polymer (after two years a significant amount of biomaterial fragments was still present) made this material unsuitable for application in biodegradable nerve guides for clinical purposes (Den Dunnen et al. (Microsurgery, 14 (1993) 508-515)). The long-lasting presence of slow-degrading biomaterial fragments could negatively affect the nerve function.
In order to increase the degradation rate and lower the risk of the formation of long-lasting biomaterial fragments, Den Dunnen et al. decided to use poly(DL-lactide-ε-caprolactone) as the material of choice (J. Biomed. Mater. Res. 29 (1995) 757-766 and Microsurgery 17 (1997) 348-357). In these publications, nerve guides are disclosed, which are based on a copolymer having a 50:50 DL-lactide:ε-caprolactone ratio. Although significant improvement was obtained with regard to the degradation rate (Den Dunnen et al. reported that the material was completely resorbed after 12 months (J. Biomed. Mater. Sci. 36 (1997) 337-346)), the material is reported to suffer from some drawbacks, such as swelling during the degradation, which may hamper the nerve regeneration, in combination with a relatively fast decrease in mechanical strength (J. Biomed. Mater. Res. 51 (2000) 575-585).
It is an object of the present invention to provide a poly(DL-lactide-ε-caprolactone) material which shows improved behavior for biodegradable medical purposes for which both flexibility and mechanical strength are prerequisites. It was found that this object can be met by providing a poly(DL-lactide-ε-caprolactone) copolymer having a specific lactide content. Therefore, in the first aspect, the present invention relates to a polymeric material comprising poly(DL-lactide-ε-caprolactone) obtained by the copolymerization of DL-lactide and ε-caprolactone, having a lactide content of 51-75 mol %, preferably of 55-70 mol %, most preferably of 62-69 mol %.
The materials of the invention show excellent mechanical properties including stress-strain behavior, modulus and tensile strength, and excellent swelling behavior, when compared to the prior art poly(DL-lactide-co-ε-caprolactone) materials. Furthermore, the materials of the invention are completely amorphous. These properties make the materials of the invention very suitable for use as degradable device for medical purposes where both flexibility and mechanical strength are important characteristics, such as nerve guides, porous scaffolds, implants, artificial skin, artificial blood vessels, films and sheets for (tissue) adhesion prevention during and after surgical operations, but also for application in implantable or injectable long-term drug delivery systems.
The lactide (cyclic ester of two lactic acid molecules) is present in the copolymer as two lactic acid units.
The materials of the present invention combine flexibility with mechanical strength up to several weeks or months after implantation. Flexibility for example, is an important pre-requisite for handling and shaping, whereas mechanical strength is important from a protective point of view. Scaffolds, for example, should provide sufficient mechanical support to allow tissue regeneration and maturation.
According to the present invention the lactide content in the copolymer is at least 51 mol %. Although copolymers with lactide contents below 51% are highly flexible, these materials have a modulus and glass transition temperature (Tg) that is too low to give the devices composed thereof sufficient mechanical strength. Moreover, the extent of swelling of these materials is too high for most biomedical applications, except possibly for sheets or wound dressings. For tubular implants or porous scaffolds for example, a low modulus and extensive swelling will lead to easier and undesired compression, deformation or total collapse and may consequently lead to insufficient protection of the newly formed tissue inside the tube.
Copolymers with a lactide content higher than 75% exhibit generally insufficient flexibility to be applied for the intended medical applications. In particular, nerve guides composed of copolymers with lactide contents higher than 75% cannot be sutured easily to the nerve stumps using an 8-0, 9-0 or 10-0 stitch, because of too high rigidity.
Therefore, the copolymers of the present invention have a lactide content of 51 to 75 mol %, preferably from 55-70 mol % and most preferably from 62-69 mol %. For example, nerve guides of 65:35 (85/15 L/D)lactide-ε-caprolactone that are used for peripheral nerve reconstruction have better mechanical properties (modulus and tensile strength) than nerve guides produced using the same monomers, but in a 50:50 monomer ratio (e.g. initial tensile strength of 40-50 MPa vs 2.5 MPa).
The known copolymers (with a low lactide content) were found to have a relatively low glass transition temperature (−12° C. vs 14° C. of the polymers according to the present invention). This low glass transition temperature affects the mechanical properties (easier compression) and swelling properties. Furthermore, polymerization conditions affect the polymer properties, as will be shown later.
Nerve guides with the indicated preferred lactide contents will retain better mechanical properties during nerve growth. The degree of swelling of these nerve guides can be very low, which prevents the risk of compression of the newly formed nerve tissue. Moreover, the flexibility of these polymers is favorable with respect to interaction with and response of the surrounding tissue
The mechanical properties and the degradation behavior, in particular swelling, of the copolymer of the present invention may further be adjusted by choosing the ratio of the L-enantiomer to D-enantiomer of the lactide in the copolymer (L/D-ratio). Preferred copolymers, having excellent mechanical properties (particularly swelling behavior), are those having a lactide L/D-ratio (mol/mol) of 65/35 to 95/5, preferably 70/30 to 90/10, most preferably about 85/15. The swelling behavior of these preferred copolymers makes them particularly suitable for use as nerve guides. Copolymers having a L/D ratio of more than 95/5 may give rise to formation of crystalline material.
Conversely, a copolymer having a lactide ratio of the D-enantiomer to L-enantiomer of the lactide (D/L-ratio) in the above-mentioned preferred ranges may be used with similar advantage. For practical reasons, however, it is preferred to prepare the polymers having the indicated L/D-ratios rather than the D/L-ratios, since the L-enantiomer is less expensive.
In particular the stress-strain characteristics are considerably improved in the above-mentioned preferred ranges, as is illustrated in FIG. 1, showing, as an example, the mechanical properties of five nerve guides prepared in accordance with the present invention at synthesis temperatures of 110° C., 120° C. and 130° C., respectively. For comparison, the stress-strain behavior of a nerve guide of a copolymer with a 50:50 monomer ratio is shown (curve G), illustrating its inferior mechanical properties compared to those with a higher lactide content.
Furthermore, the L/D ratio has a pronounced effect on the swelling behavior, which is illustrated in FIG. 4. This figure clearly shows that swelling is reduced in nerve guides of a copolymer with a L/D ratio in the preferred range, compared to those of a copolymer with a L/D ratio equal to unity, while having the same lactide/caprolactone ratio.
The polymeric material according to the present invention may be produced by a conventional copolymerization reaction, as will be further illustrated hereinbelow. One of the synthesis conditions is the co-polymerization temperature. It was found that copolymers produced at 110° C. show somewhat better initial mechanical properties than copolymers with a similar composition and produced at 120° C.
A polymerization temperature lower than 110° C. will result in a lower conversion and a considerable longer polymerization time to obtain high conversions. Also, homogeneous mixing of the monomers is more difficult at lower temperatures (lactide melts at about 125° C.).
Although a high conversion generally will give a copolymer of a higher purity, the required purity can also be obtained easily by extraction of unreacted monomers using organic solvents.
The composition of the prepared copolymer may be determined using 1H-NMR at 300 MHz of solutions of the copolymer in deuterated chloroform. It is noted that the relative starting amounts of lactide and ε-caprolactone monomers used to synthesize the copolymer may differ significantly from the amount that is actually incorporated into the copolymer, as a result of incomplete conversion. The actual amount of monomers in the copolymers of the present invention (e.g. expressed as lactide content in the polymer, rather than relative amount of starting materials) can always be assessed by 1H-NMR at 300 MHz of solutions of the copolymer in deuterated chloroform.
The intrinsic viscosity may be used as a parameter which reflects the molecular weight, as will be explained in more detail below.
Preferred temperature ranges for the copolymerization reaction are from 100° C.-130° C., more preferably from 110-120° C. A higher polymerization temperature, generally results in a higher conversion but a lower molecular weight and shorter sequences of monomers in the copolymer.
The distribution of monomers in the copolymer may have a large effect on its properties. Since lactide and ε-caprolactone have different reactivities, the copolymer will have a more or less blocky structure. The length of the copolymer units consisting of one type of monomer (average sequence length, L;  Lac and L;  Cap) is determined by the polymerisation conditions and monomer composition: higher polymerisation temperature, longer polymerization time and higher catalyst concentration result in smaller average sequence lengths (more transesterification takes place). A larger content of one of both monomers in the polymer will result in a larger average sequence length of this monomer.
Also, the L/D ratio of the lactide effects the average sequence lengths and therefore the other polymer properties (see results presented in Table 2).
The average lactide sequence length is generally higher in copolymers with a L/D ratio away from unity, such as L/D=85/15 (see e.g. Examples C and D below) than in similar copolymers with a L/D ratio equal or close to unity, such as L/D=50/50 (see e.g. Examples E and F below), when prepared under the same polymerisation conditions. Since the reactivity of L- and D-Lactide is the same, the average sequence length must be determined by the transesterification rate, which is lower in copolymers with L/D=85/15 than in copolymers with L/D=50/50, under the same conditions.
A suitable polymerization time is co-determined by the other polymerization conditions, in particular by the polymerization temperature and amount of catalyst used. Generally, the polymerization time ranges from 3-40 days, preferably from 5-13 days. Also, a longer polymerization time generally results in a higher conversion but a lower molecular weight and smaller sequences of monomers in the polymer. With a catalyst concentration of M/I=9000-12000 the preferred polymerisation time and temperature vary from a minimum of 3 days at 130° C. to a maximum of 40 days at 100° C. With lower catalyst concentration, the polymerisation time must be longer at the same temperature.
As mentioned before, as an example, nerve guides of 65:35 (85/15 L/D)lactide-ε-caprolactone that are used for peripheral nerve reconstruction have better mechanical properties than nerve guides with a 50:50 monomer ratio. Both the polymer composition and the method of preparation of the copolymer determine the mechanical properties of the nerve guides: a low lactide content (50%) and a very long polymerisation time of 22 days at 130° C. will result in a complete transesterification of the monomers, giving smaller average monomer sequence lengths. In an ideal random 50:50 copolymer the average lactide and caprolacton sequence length, L;  Lac and L;  Cap, will be 4 and 2, respectively (H. R. Kricheldorf and I. Kreiser, J. Macromol. Sci. Chem., A24 (11), 1345, (1987)). The monomer distribution in the prior art copolymer is completely random. Therefore, the small average lactide sequence length will result in poorer mechanical properties (e.g. modulus, tensile strength or toughness) of the 50:50 copolymer compared to those of the copolymers with a larger lactide content (and thus with a longer average lactide sequence length).
Ultimately, after prolonged polymerisation time and/or high polymerisation temperature, the monomer distribution will be completely random in all cases (all L/D ratios). However, of two copolymers with a similar average lactide sequence length, the one with a L/D ratio away from unity (e.g. 85/15) shows better mechanical properties than the one with a L/D ratio equal to unity. This proves that not only the size of the average monomer sequence lengths, but also the L-lactide content of the lactide sequence determines the mechanical properties. These results are confirmed by the data shown in Table 2.
Another important property of the copolymers of the present invention is the molecular weight and in particular the molecular weight distribution. The molecular weight (distribution) may e.g. be determined using gel permeation chromatography (GPC). More conveniently, however, the molecular weight is determined using a standardized intrinsic viscosity assay (ISO 1628-1). The value for the intrinsic viscosity ([ ]) thus obtained (expressed in dl/g) may be converted into the weight average molecular weight (Mw) by using the well-known Mark-Houwink expression: [ ]=KMwa, wherein K and a are polymer specific parameters. For lactide-ε-caprolactone copolymers having a lactide content of 45-55% the Mark-Houwink constants were determined experimentally using GPC: K=3.303 10−3 and a=0.548. Similarly, the number-average molecular weight (Mn) may be correlated to the intrinsic viscosity: [ ]=1.019 10−3·Mn0.659, wherein the parameters were also determined using GPC. These values can also be applied to the copolymers of this invention with a higher lactide content.
Preferred polymeric materials of the present invention are those having a molecular weight that corresponds to an intrinsic viscosity, as assessed as described above, of more than 4 dl/g, preferably of more than 5 dl/g, more preferably of more than 5.5 dl/g. The molecular weight distribution or polydispersity (Mw/Mn), as determined using GPC) is preferably less than 1.8, more preferably less than 1.5.
The properties of the polymeric material, in particular the molecular weight (distribution), may be controlled by varying the amount of catalyst used in the polymerization reaction. Particularly good results are obtained by employing the catalyst in an amount such that the molar ratio of lactide and ε-caprolactone monomers to the catalyst (M/I) is from 1000-100000, preferably from 9000-12000. Lower M/I ratios (corresponding to a high catalyst concentration) generally result in faster polymerization and higher conversion, resulting in lower molecular weight and shorter sequences of monomers in the polymer. Higher M/I ratios generally give a higher molecular weight and longer sequences of monomers in the polymer. Suitable catalysts for this purpose are known to the skilled person. Particularly suitable are Sn(oct)2 catalysts.
Of course, since the above-mentioned parameters (including polymerization time, polymerization temperature, initial monomer ratio and catalyst concentration) are correlated, optimal values for each of these parameters will differ per case. These optimal values can, however, easily be found in carrying out the invention, optionally by carrying out some routine experiments.
The copolymers of the present invention may be used in a variety of applications, in particular to provide the above-mentioned degradable devices for medical purposes, particularly to provide nerve guides. The polymers of the present invention can e.g. be processed into films, sheets, tubes, rods, plugs, microspheres or meshes, either solid or porous. Pores may vary from small and non-interconnected to large and interconnected. Microporous films (membranes) may also be produced of this material (e.g. having pore sizes as small as 5 μm).
Examples of other products that can be made of the materials of this invention are, biomedical drains; biomedical tubes for stenting purposes; biomedical sheets for intra-corporeal application such as an anti-adhesive sheet; biomedical sheets for topical use such as temporary wound care coverage or to prevent scarring; wound care foams; protective sheaths for needles and tubes which are introduced into the body; (micro)spheres for drug delivery; (micro)spheres, particles and plugs for embolization purposes; (micro)spheres for cosmetic surgical purposes, such as dermal augmentation, treatment of wrinkles and skin contour deficiencies; vascular prostheses; tissue engineering scaffolds, such as artificial skin or scaffolding plug for meniscus repair.
As will be illustrated in the examples below, the materials of the present invention have excellent properties, which include mechanical strength (tensile strength, modulus, strain at break, suture strength), thermal properties (glass transition temperature, crystallisation) and swelling (water uptake, increase of volume). Important for application as a nerve guide is the retention of its mechanical strength and compressibility during the time the new nerve has matured and retention of its dimensions (some swelling may be desirable for obtaining a permeable product, however, too much swelling results in obstruction of the lumen which will negatively affect nerve fiber regeneration).